This invention relates generally to computer-controlled automatic test systems for testing integrated circuit and discrete devices, and more particularly to a plug-in intelligent test head adapted to execute special tests on an existing automatic test system which it is not otherwise capable of carrying out.
In the large-scale production of modern electronic equipment, component inspection is mandatory. In recent years, the tendency in quality control procedures is away from sample testing of components and toward 100% inspection. The practical reason for this is that some lots of components, when individually tested, may contain as many as 5% rejects. Merely sampling a lot of this character will not extract all bad components; hence bad components will go into the production line, and these must be identified and replaced during or after production.
Experience has demonstrated that finding a bad component at a board test is about ten times more expensive than finding it during an incoming inspection; whereas finding it in a system test costs 100 times as much. And if the bad part is not located until the equipment reaches the field, it will cost about 1,000 as much to find and replace it.
A major function, therefore, of commercially-available computer-controlled automatic test systems which make possible 100% incoming inspection of integrated circuit and discrete devices is the acquisition of the best quality devices for a given application at the lowest possible price. Typical of automatic test systems of the type now available is the Teradyne J 283 circuit test system manufactured by Teradyne, Inc. of Boston, Mass. The J 283 test system is capable of testing almost all types of digital integrated circuits, hybrid devices and other forms of digital modules and printed circuit boards. This tester consists of three major elements; namely, a computer controller with its associated input/output equipment and software; the main frame; and one or more satellites, each satellite being coupled to various test stations.
In addition to a range of automatic test systems being marketed by Teradyne, test systems are also available from Fairchild, Lorlin Industries, LTX and other companies. These are designed to test either linear or analog circuits, or digital circuits, but never both types with any degree of flexibility. The primary reason why all of the possible functions necessary to carry out testing on both linear and digital circuits are not incorporated in a single automatic test system of the type presently available is cost; for a main frame tester of the above type has a selling price that runs as high as $750,000. Moreover, these computer-controlled testers require extensive software support which can easily multiply the initial cost by a factor of three or four.
The main concern of the present invention is the inability of commercially-available computer-controlled automatic test systems to perform one or several specialized or non-standard tests that lie beyond their existing capabilities. As a consequence, an electronic equipment manufacturer may have a half-million dollar automatic test system on his production floor capable of testing a family of IC devices and yet be unable to perform certain essential tests on his supply of components.
One approach heretofore taken by manufacturers toward solving this problem is to design in-house and construct "stand-alone" systems and hand-insert the integrated circuit devices therein so that the necessary specialized tests may be run on the devices. Because of the substantial investment entailed in manpower, time and capital equipment in order to effect testing in this manner, this in-house approach leaves much to be desired from the production standpoint and in economic terms.
Another approach heretofore attempted by some manufacturers who have made a heavy investment in an automatic test system that is incapable of carrying out certain nonstandard yet necessary tests, is to install special electronic modules on the performance boards of the main frame tester and to use the main frame tester to control, to acquire and to process data from the modules. In general, these attempts have not produced satisfactory results. This is because of the basic structure of the main frame tester. In the software or system architecture of existing computer-controlled automatic test systems, no allowances are made for the additional commands that it would take to drive special electronic modules connected to the pin electronics of the performance boards. By reason of this limitation, it is not a simple or easy matter to build a test module and integrate it with the main frame tester.
Accordingly, the present invention is directed to both the problem of economically expanding the test capabilities of a main frame test computer and the problem of control and data usage within the main frame with a minimum of software rewrite.
Another serious limitation of existing automatic digital test systems, such as the Fairchild "Sentry" and certain Teradyne testers, is that in these testers the digital stimulus for the device under test in some cases dictates that very high currents be sequentially switched on and off at relatively high rates, thereby giving rise to electrical noise.
Because of current noises generated in this manner in the test instrument, it is usually impossible, as a practical matter, to operate a digital-based main frame tester to carry out any kind of low level analog or linear testing; for the signal-to-noise ratio is then unfavorable. The current switching pulses in such test arrangements are usually fed through the electrical conductors, and the resultant inductive and/or transient noises act to mask or degrade precision low-level measurement.
To illustrate the limitations of existing automatic techniques for testing integrated circuits, we shall now consider the problem of testing a basic, integrated-circuit device used in telecommunications. This device is the integrated-circuit "codec" chip or coder-decoder.
As pointed out in the article on "IC's for Telecommunications," appearing in Electronics Design, Apr. 12, 1979, codec chip sets are probably the most technically demanding of all telecom IC's; for here the analog and digital worlds of communication come together. At the channel's transmission end, a codec serves to encode or translate the analog voice signal into an appropriate PCM code; while at the receiving end, a codec decodes and thereby reforms voice signals from the transmitted digital signal.
One procedure heretofore practiced in testing codec's is to carry out digital testing of the codec chip on a digital main frame test computer and then remove the chip and transfer it into an analog instrument test rack. The total hardware cost involved in providing separate and distinct digital and analog testers for this purpose may run as high as a million dollars. An alternative practice is for the manufacturer to purchase a specialized tester capable only of testing codec's, but this tester generally costs over $300,000. Because existing general purpose computer-controlled digital test systems are incapable of carrying out all of the requisite tests on a codec chip, the testing of such chips has hitherto been, for the reasons explained above, a highly expensive proposition.